1. Field of the Invention
The present invention relates to a copolyester film suitable for use as a hot melt adhesive and a method of producing the same.
2. Description of the Background
In recent years, hot melt adhesives have been used in increasing quantities due to their non-polluting, energy-saving and resource-conservation characteristics. Hot melt adhesives of the film type in particular, are known to be advantageous in that they produce an adhesive layer of uniform thickness and have a stable, uniform bond strength. Additionally, the desired adhesion can be established by mere melting of the superficial layer of the film. They lend themselves well to a short-time bonding process and require a minimum of bonding heat. Moreover, the thermal degradation of the adhesive is negligible.
Hot melt adhesives of the polyester type are well known. These adhesives are excellent in heat and cold resistance, thermal stability, electrical properties, weather resistance, moisture resistance, resistance to chemicals and safety in food use, and have therefore been widely used in such applications as electrical parts assembly, automotive parts assembly, textiles, can manufacture, etc.
Many proposals have heretofore been made regarding the components of polyester hot melt adhesives. For example, a hot melt adhesive of polyesters derived from a selected dicarboxylic acid component and a selected diol component having special structural formulas as the polyester materials is well known. Also known is a hot melt adhesive of polymers having a carboxylic acid amide, carboxylic acid anhydride, carbonic acid ester, urethane or urea bond either in the backbone chain or in the side chain (Japanese patent publication No. 22750/82). It is possible to improve a given property by using selected components or a selected combination of components, but such attempts tend to result in changes in other physical properties as well.
U.S. Pat. No. 4,143,790 discloses a polyester hot melt adhesive in which at least 45 mole % of the dicarboxylic acid component is accounted for by terephthalic acid and at least 55 mole % of the diol component is accounted for by 1,4-butanediol. Its relative viscosity and degree of crystallinity are in the ranges of 1.2 to 1.8 and 0 to 30 percent, respectively.
U.S. Pat. No. 3,515,628 teaches a polyester copolymer-based hot melt adhesive in which the dicarboxylic acid component is a mixture of terephthalic acid and an aliphatic dicarboxylic acid and its glycol component is either 1,4-butanediol or a mixtue of 1,4-butanediol and ethylene glycol, neopentyl glycol or the like. The adhesive has a glass transition temperature not higher than 30.degree. C. and a degree of crystallinity not exceeding 25%.
U.S. Pat. No. 4,335,188 describes a copolyester hot melt adhesive in which at least 85 mole % of its dicarboxylic acid component is terephthalic acid and its diol component is a mixture of 97 to 65 mole % 1,4-butanediol and 3 to 35 mole % polyethylene glycol having a molecular weight in the range of 150 to 550.
Furthermore, U.S. Pat. No. 4,500,575 discloses a hot melt adhesive comprising a polyester copolymer consisting of a dicarboxylic acid component made from 70 to 97 mole % terephthalic acid and the balance of the other dicarboxylic acids, and a diol component made from 70 to 97 mol % 1,4-butanediol and 3 to 30 mol % diethylene glycol, with a reduced viscosity of a least 0.6 dl/g, a glass transition temperature of not less than 31.degree. C., a degree of crystallinity in the range of 15 to 40% and a creep displacement of not greater than 0.16 mm.
The present inventors have previously filed a patent application (U.S. Pat. Nos. 730,055, 4,659,615 EP-85303366.0) for an invention directed to a technique for accomplishing well-coordinated tensile shear bond strength, T-peeling bond strength, durability and high-speed adhesion which had never been obtained from the hot melt adhesives of the prior art technologies which have been described briefly above
However, the film obtainable from a copolyester according to the usual commercial film forming technology proposed thus far has a low initial modulus of elasticity at elevated temperatures (30.degree. C. to 50.degree. C.) and stretches under low tension. This leads to the disadvantage that with an increase in temperature in the production environment, the film is liable to stretch, causing a deterioration in workability in the film forming stage of production.